1. Field of the Invention
The present invention relates to an improvement of an image forming lens having a single focal length, employed as an optical system for photographing in various types of cameras including a so-called silver salt camera, in particular, cameras such as a digital camera, a video camera and so on, and more specifically also relates to an image forming lens suitable for the digital camera, video camera and so on, as well as a camera and a portable information terminal using such the image forming lens.
2. Description of the Related Art
A camera of a type which images a image of an object to be photographed with a solid-state image pick-up device such as a CCD (Charge-Coupled Device) image pick-up device or the like, acquires image data(still image) or a motion image (movie image) of the photographic object, and digitally stores the acquired image data into a nonvolatile semiconductor memory or the like represented by a flash memory, typically referred to as a digital camera or an electronic camera etc., has been generalized in recent years.
A market for such a digital camera has been a very large one, and the user's demands for the digital camera have been ranged in scope. Among others, the digital camera having a zooming function has been popular, a category of “a compact camera of small and high image quality on which an image forming lens having a high performance and a single focus optical system is mounted” has been gained a constant support from the user. As the user's demand, in addition to the camera having a high performance, a F value (number) (hereinafter, referred to “F”) is small, that is to say, the weight that the optical system has a large diameter has been become higher.
A market for such a digital camera has been a very large one, and the user's demands for the digital camera have been high and ranged in scope. More specifically, from the viewpoint of the aspect of “high performance” in the compact camera, in addition to, it has at least a resolution for the image pick-up device having from one thousand to two thousands pixels.
“From the opening of a diaphragm aperture, there are little Coma flares and thus the lens has a high contrast while there are no point image collapse up to the periphery of the field angle of view”,
“There are few chromatic aberration and there are no colors even in a portion where large brightness differences exist”, and
“Distortional aberration is small and that it is possible to give a vivid description of a line” and the like are required.
Further, in the aspect of “a large diameter”, from the standpoint from patentably differentiating from a general compact camera which mounts a zooming lens, it is required to be at least less than F=2.4, further, it is desired to be less than F=2.0 to no small extent
With regard to “field angle of view of a photographing lens”, since there are many users who wish to desire a wide angle to a certain extent, it is desirable to make the field angle of view of the image forming lens more than 38 degrees (which corresponds to a focal length of 28 mm which is converted to a 35 mm silver salt camera or the so-called Leica version).
Although there considered a variety of image forming lens for digital cameras, as a typical arrangement of a wide angular single focal length lens, there exists a so-called retrofocus type lens system having a first lens group with negative power which is positioned in the object side and a second lens group with positive power which is positioned in the image side.
Based upon the property of an area sensor which has color filter(s) or micro lens(s) every pixel, there exists that the position of the exit pupil is away from an image surface and thus the peripheral luminous flux is incident at an angle substantially perpendicular to a light-receiving surface of the area sensor. This is why the retrofocus type lens system is employed.
However, since the retrofocus type lens in which cardinal points are positioned behind the lens system has a large asymmetric refractive power.
Among such prior art retrofocus type image forming lenses, as the image forming lens having a relatively large diameter and field angle of view of about 38degrees, each of JP-H06-308385-A, JP-A-H09-218350, and JP-2006-349920-A discloses such the image forming lens.
However, although an image forming lens disclosed in JP-H06-308385-A has a large aperture of F=1.4, there are large astigmatism or field curvature and it is difficult to say to have “a sufficient performance up to the periphery”.
An image forming lens disclosed in JP-H09-218350-A is inferior to the recent requirement level as being F=2.8 in enlarging a diameter and thus astigmatism, field curvature and chromatic aberration of magnification are large, so it is not difficult to say that they are not sufficiently corrected and controlled.
Accordingly, it goes without saying that this image forming lens has a “sufficient performance up to the peripheral portion”.
Furthermore, distortional aberrations of both exceed 2% in the absolute value and thus not satisfactory in this respect.
Further, JP-2006-349920-A discloses an image forming lens in which astigmatism, field curvature, and distortional aberration are sufficiently corrected and controlled but color difference of coma aberration, which is not shown throughout the specification is large.
Further, an Example in which an F value (number) is small, is not sufficient in miniaturization, and thus further improvements are necessary.